Under the guidance of the NANDS Council and under the aegis of the National Eye Institute, a cooperative study involving a number of centers throughout the country for the systematic study of diabetic retinopathy has been proposed. In addition to documenting, with as great statistical precision as possible, the natural history of diabetic retinopathy and the prognosis without treatment, it is proposed to determine whether photocoagulation treatment of the eye with retinopathy is useful in preserving vision. Moreover, it is hoped to learn whether modifications of treatment technique (xenon vs. laser) are of clinical importance and if the local treatment has an effect on other areas other than those specifically treated. It is proposed to set up a multi- center collaborative study to acquire enough data to achieve statistical significance with the total number of patients to be well over 1,000. Patients from the various medical centers will represent different geographical areas and socioeconomic strata. Patients with visual acuity of 20/100 or better in each eye with proliferative diabetic retinopathy or moderately advanced background retinopathy in at least one eye will be used for this study. One eye will be randomly assigned treatment and the other followed as the untreated control. Photographs of seven standard areas of the fundus will be made prior to treatment, and at twenty-four hours, four months, and 12 months after treatment and at annual intervals thereafter. In addition, appropriate medical tests will be done and patients will be appropriately treated for their diabetes mellitus.